In a manufacturing process of a semiconductor device, a fine pattern is generally formed using a photolithographic method. In the formation of the fine pattern, multiple substrates, which are referred to as transfer masks, are usually used. The transfer mask is formed by providing the fine pattern comprised of a metal thin film, etc. on a generally transparent glass substrate. The photolithographic method is also used in the manufacture of the transfer mask.
Refinement of a pattern for the semiconductor device requires shortening of a wavelength of an exposure light source used in photolithography, in addition to the refinement of a mask pattern formed in the transfer mask. Nowadays, the exposure light sources used in the manufacture of semiconductor devices are shifting from KrF excimer lasers (wavelength: 248 nm) to ArF excimer lasers (wavelength: 193 nm), that is, shorter wavelength light sources are increasingly used.
The known types of transfer masks include a half tone phase shift mask, in addition to a conventional binary mask including a light shielding film pattern made of a chromium-based material on a transparent substrate. The half tone phase shift mask includes a light-semitransmissive film pattern on the transparent substrate. The light-semitransmissive film (half tone phase shift film) has functions to transmit light at an intensity not substantially contributing to the light exposure and to generate a predetermined phase difference between the light transmitted through the light-semitransmissive film and the light transmitted through the air for the same distance, thereby generating a so-called phase shift effect.
Generally, an outer peripheral region of the transfer mask outside the region where a transfer pattern is formed is required to ensure optical density (OD) of not less than a predetermined value such that, upon the exposure transfer to a resist film on a semiconductor wafer using an exposure apparatus, the resist film will not be affected by the exposure light transmitted through the outer peripheral region. Usually, the outer peripheral region of the transfer mask desirably has OD of 3 or more, and at least about 2.8 of OD is necessary. However, the light-semitransmissive film of the half tone phase shift mask has a function to transmit the exposure light at a predetermined transmittance, and it is difficult to ensure the optical density required in the outer peripheral region of the transfer mask with the light-semitransmissive film alone. Therefore, as with a phase shift mask blank disclosed in Patent Document 1, a light shielding film (light blocking film) is laminated onto a semitransparent film having predetermined phase shift amount and transmittance with respect to the exposure light to ensure the predetermined optical density in a laminated structure of the semitransparent film and the light shielding film.
There is also a phase shift mask blank as disclosed in Patent Document 2, in which a light shielding film provided on a phase shift film is made of a material containing a transition metal and silicon. In this phase shift mask blank, the material containing a transition metal and silicon is also used as a material for forming the phase shift film, as is conventionally done. Therefore, it is difficult to ensure etching selectivity between the phase shift film and the light shielding film in the dry etching. The phase shift mask blank of Patent Document 2 includes, between the phase shift film and the light shielding film, an etching stopper film made of a material containing chromium. It further includes, on the light shielding film, an etching mask film made of the material containing chromium.